Multi-leaf collimators (MLCs) are used in radiotherapy for aperture forming intended to shape a radiation beam. Examples of multiple layer MLCs include those described in U.S. Pat. Nos. 6,266,393 and 6,526,123 and to Ein-Gal, the disclosures of which are incorporated herein by reference. MLCs provide a stationary aperture for radiotherapy.
Dynamic aperture formation is also known in the art. For example, U.S. Pat. No. 4,868,843 to Nunan describes a system that dynamically controls an x-ray field of a fan beam. A multileaf collimator is positioned in the fan beam including a first set of leaves which can be individually moved into or out of the fan x-ray beam to block or pass individual radiation pixels. Continuous monitoring of alignment of the patient's anatomy with both inner and outer edges of the fan beam is obtained with a linear detector array retractably mounted on the opposite side of the patient from the x-ray source. Tapered extensions, added to a second opposite set of leaves of the MLC are variably positionable to attenuate the dose rate in individual radiation pixels of the fan x-ray beam. The patient scan is obtained by moving the patient perpendicularly to and through the fan x-ray field while the dose delivered in each radiation pixel is dynamically controlled. Normal tissue is protected by the positions of the first set of leaves of the MLC, which attenuate transmission to less than 5% of open field dose. Depth variations from the patient surface to the plane at treatment depth are compensated at each radiation pixel of the field by the positions of the tapered extensions of the second, opposite, set of leaves of the MLC, providing variable transmission from 50% to 100% of open field dose, for example. Reduced dose to critical organs such as the spinal cord can thereby be delivered in each treatment fraction
However, dynamic aperture formation for target tracking requires leaf speed significantly higher than presently available. Increasing MLC leaf speed is presently complicated and expensive.